Implement events for sync status (#353)

`y-websocket` has a `synced` flag, but it is a bit misleading because it
only tells whether the initial handshake was completed. Knowing whether
a document was synced _in general_ for `y-protocols` is a bit tricky,
because we would need to compare the state vector and delete set.

When using WebSockets, we can take advantage of the ordering guarantee
and synchronous message processing in Y-Sweet to implement a very simple
check of sync.

The client stores two numbers, `lastSyncSent` and `lastSyncAcked`. When
it sends an update to the server, it also increments `lastSyncSent` and
then sends a separate `messageSyncStatus` message containing that number
as the payload.

When the server receives a `messageSyncStatus`, it simply echoes it
verbatim. When the client receives the `messageSyncStatus` in return, it
updates `lastSyncAcked` to the payload of the message.

If `lastSyncAcked` = `lastSyncSent`, we know that all messages since the
last update have been processed. Otherwise, there are outstanding
changes.

This builds on #352 and will enable #306.

Demo:


https://github.com/user-attachments/assets/60d4aec2-f025-4e84-a594-28fbc84c67cb

crates/y-sweet/src/server.rs

@@ -17,7 +17,6 @@ use futures::{SinkExt, StreamExt};
 use serde::Deserialize;
 use serde_json::{json, Value};
 use std::{
-    net::SocketAddr,
     sync::{Arc, RwLock},
     time::Duration,
 };

examples/nextjs/src/app/(demos)/color-grid/ColorGrid.tsx

@@ -14,7 +14,7 @@ export function ColorGrid() {
   const [color, setColor] = useState<string | null>(COLORS[0])
 
   return (
-    <div className="space-y-3 p-4 lg:p-8">
+    <div className="space-y-3">
       <Title>Color Grid</Title>
       <div className="space-x-2 flex flex-row">
         {COLORS.map((c) => (

examples/nextjs/src/app/(demos)/color-grid/page.tsx

@@ -1,12 +1,16 @@
 import { YDocProvider } from '@y-sweet/react'
 import { randomId } from '@/lib/utils'
 import { ColorGrid } from './ColorGrid'
+import StateIndicator from '@/components/StateIndicator'
 
 export default function Home({ searchParams }: { searchParams: { doc: string } }) {
   const docId = searchParams.doc ?? randomId()
   return (
     <YDocProvider docId={docId} setQueryParam="doc" authEndpoint="/api/auth">
-      <ColorGrid />
+      <div className="p-4 lg:p-8">
+        <StateIndicator />
+        <ColorGrid />
+      </div>
     </YDocProvider>
   )
 }

examples/nextjs/src/components/StateIndicator.tsx

@@ -0,0 +1,29 @@
+'use client'
+
+import { STATUS_CONNECTED } from '@y-sweet/client'
+import { useConnectionStatus, useHasLocalChanges } from '@y-sweet/react'
+
+export default function StateIndicator() {
+  let connectionStatus = useConnectionStatus()
+  let hasLocalChanges = useHasLocalChanges()
+
+  let statusColor = connectionStatus === STATUS_CONNECTED ? 'bg-green-500' : 'bg-red-500'
+  let syncedColor = hasLocalChanges ? 'bg-red-500' : 'bg-green-500'
+
+  return (
+    <div className="mb-4">
+      <div className="flex flex-row items-center text-xs space-x-1 w-fit bg-white rounded-md p-1 text-gray-500">
+        <div>CONNECTED:</div>
+        <div
+          className={`w-3 h-3 rounded-full transition-colors ${statusColor}`}
+          title={connectionStatus}
+        ></div>
+        <div>SYNCED:</div>
+        <div
+          className={`w-3 h-3 rounded-full transition-colors ${syncedColor}`}
+          title={hasLocalChanges ? 'Unsynced local changes.' : 'No unsynced local changes.'}
+        ></div>
+      </div>
+    </div>
+  )
+}

js-pkg/client/src/main.ts

@@ -1,7 +1,31 @@
-import { YSweetProvider, type YSweetProviderParams, type AuthEndpoint } from './provider'
-import * as Y from 'yjs'
 import { ClientToken, encodeClientToken } from '@y-sweet/sdk'
-export { YSweetProvider, YSweetProviderParams, AuthEndpoint }
+import * as Y from 'yjs'
+import {
+  type AuthEndpoint,
+  EVENT_CONNECTION_STATUS,
+  EVENT_LOCAL_CHANGES,
+  STATUS_CONNECTED,
+  STATUS_CONNECTING,
+  STATUS_ERROR,
+  STATUS_HANDSHAKING,
+  STATUS_OFFLINE,
+  YSweetProvider,
+  type YSweetProviderParams,
+  type YSweetStatus,
+} from './provider'
+export {
+  AuthEndpoint,
+  EVENT_CONNECTION_STATUS,
+  EVENT_LOCAL_CHANGES,
+  STATUS_CONNECTED,
+  STATUS_CONNECTING,
+  STATUS_ERROR,
+  STATUS_HANDSHAKING,
+  STATUS_OFFLINE,
+  YSweetProvider,
+  YSweetProviderParams,
+  YSweetStatus,
+}
 
 /**
  * Given a docId and {@link AuthEndpoint}, create a {@link YSweetProvider} for it.